CUSS K. Drage, Ed. Internet-Draft Alcatel-Lucent Intended status: Standards Track A. Johnston Expires: April 27, 2015 Avaya October 24, 2014 Interworking ISDN Call Control User Information with SIP draft-ietf-cuss-sip-uui-isdn-10 Abstract The motivation and use cases for interworking and transporting ITU-T DSS1 User to User information element data in SIP are described in RFC 6567. As networks move to SIP, it is important that applications requiring this data can continue to function in SIP networks as well as the ability to interwork with this ISDN service for end-to-end transparency. This document defines a usage (a new package) of the User-to-User header field to enable interworking with this ISDN service. This document covers the interworking with both public ISDN and private ISDN capabilities, so the potential interworking with QSIG will also be addressed. The package is identified by a new value "isdn-uui" of the "purpose" header field parameter. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. 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." This Internet-Draft will expire on April 27, 2015. Copyright Notice Copyright (c) 2014 IETF Trust and the persons identified as the Drage & Johnston Expires April 27, 2015 [Page 1] Internet-Draft ISDN Call Control UUI October 2014 document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) 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. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Summary of the ISDN User to User Service . . . . . . . . . . . 3 3.1. The service . . . . . . . . . . . . . . . . . . . . . . . 3 3.2. Impacts of the ISDN service on SIP operation . . . . . . . 5 4. Relation to SIP-T . . . . . . . . . . . . . . . . . . . . . . 6 5. Transition away from ISDN . . . . . . . . . . . . . . . . . . 6 6. ISDN Usage of the User-to-User Header Field . . . . . . . . . 7 7. UAC requirements . . . . . . . . . . . . . . . . . . . . . . . 8 8. UAS requirements . . . . . . . . . . . . . . . . . . . . . . . 9 9. UUI contents . . . . . . . . . . . . . . . . . . . . . . . . . 10 10. Considerations for ISDN internetworking gateways . . . . . . . 11 11. Coding requirements . . . . . . . . . . . . . . . . . . . . . 12 12. Media Feature Tag . . . . . . . . . . . . . . . . . . . . . . 12 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 14. Security Considerations . . . . . . . . . . . . . . . . . . . 14 15. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 14 16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15 16.1. Normative References . . . . . . . . . . . . . . . . . . . 15 16.2. Informative References . . . . . . . . . . . . . . . . . . 15 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16 Drage & Johnston Expires April 27, 2015 [Page 2] Internet-Draft ISDN Call Control UUI October 2014 1. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. 2. Overview This document describes a usage of the User-to-User header field defined in [I-D.ietf-cuss-sip-uui] to enable the transport of User to User Information (UUI) in ISDN interworking scenarios using SIP [RFC3261]. Specifically, this document discusses the interworking of call control related ITU-T DSS1 User to User information element Q931 [Q931], Q957.1 [Q957.1] and ITU-T Q.763 User to User information parameter [Q763] data in SIP. UUI is widely used in the PSTN today in contact centers and call centers which are transitioning away from ISDN to SIP. This usage is not limited to scenarios where interworking will occur. Rather it describes a usage where interworking is possible if interworking is met. That does not preclude its usage directly between two SIP terminals. 3. Summary of the ISDN User to User Service 3.1. The service ISDN defines a number of related services. Firstly there is a user signalling bearer service, which uses the information elements / parameters in the signalling channel to carry the data, and does not establish a related circuit-switched connection. For DSS1, this is specified in ITU-T Recommendation Q.931 section 3.3 and section 7 [Q931]. It also defines a User to User signalling supplementary service, which uses the information elements / parameters in the signalling channel to carry additional data, but which is used in conjunction with the establishment of a related circuit-switched connection. This reuses the same information elements / parameters as the user signalling bearer service, with the addition of other signalling information, and for DSS1 this is specified in ITU-T Recommendation Q.957.1 [Q957.1]. ISDN defines three variants of the User to User signalling supplementary service as follows: Drage & Johnston Expires April 27, 2015 [Page 3] Internet-Draft ISDN Call Control UUI October 2014 UUS1: User to User information exchanged during the setup and clearing phases of a call, by transporting User to User information element within call control messages. This in itself has two subvariants, UUS1 implicit and UUS1 explicit. In UUS1 implicit, it is the presence of the user signalling data itself that constitutes the request for the service. UUS1 explicit uses additional supplementary service control information to control the request and granting of the service, as in UUS2 and UUS3. UUS1 explicit as a result also allows the requester to additionally specify whether the parallel circuit-switched connection should proceed if the UUS1 service cannot be provided (preferred or required); UUS2: User to User information exchanged from the sender's point of view during call establishment, between the DSS1 ALERTING and DSS1 CONNECT messages, within DSS1 USER INFORMATION messages; and UUS3: User to User information exchanged while a call is in the Active state, within DSS1 USER INFORMATION messages. The service is always requested by the calling user. This document defines only the provision of the ISDN UUS1 implicit supplementary service to interworking scenarios, this being the most widely deployed and used of the various ISDN User to User services, and indeed the one that matches the requirements specified in RFC6567 [RFC6567]. The above come from the ISDN specifications defined for public networks. There are a parallel set of ISDN specifications defined for private networks (QSIG}. These specifications do not define a UUS1 implicit supplementary service. However, implementation of such a UUS1 implicit supplementary service for private networks can readily be constructed in a proprietary fashion based on the specifications for public networks, and evidence suggests that some vendors have done so. On this basis, there is no reason why this package cannot also be used to support interworking with such a private network service, on the assumption that the constraints are exactly the same as those for the public network. The ISDN UUS1 service has the following additional characteristics as to the data that can be transported: The maximum number of octets of user information that can be transported is 128 octets plus a protocol discriminator. It is noted that some early ISDN implementations had a limitation of 32 octets, but it is understood that these are not currently deployed. While this package does not prohibit longer data Drage & Johnston Expires April 27, 2015 [Page 4] Internet-Draft ISDN Call Control UUI October 2014 fields, the mechanism at any interworking point is to discard data elements that are too long to handle. The handled length can normally be assumed to be 128 octets. The content of the user information octets is described by a single octet protocol discriminator (see table 4-26 of ITU-T Recommendation Q.931) [Q931]. That protocol descriminator may describe the protocol used within the user data, the structure of the user data, or leave it entirely open. Note that not all values within the protocol discriminator necessarily make sense for use in the User to User service, as the content is aligned with the protocol discriminator that appears at the start of all DSS1 messages (see table 4-1 of ITU-T Recommendation Q.931) [Q931]. The protocol discriminator value has no impact on the interworking capability. Only a single user information can be transported in each message. The ISDN service works without encryption or integrity protection. The user trusts the intermediate network elements, and therefore the operator of those elements, not to modify the data, and to deliver all the data to the remote user. On a link by link basis, message contents are protected at layer 2 by standard CRC mechanisms - this allows loss on a link level basis to be detected, but does not guard against fraudulent attacks on the link itself. This does not prevent the use of additional encryption or integrity protection within the UUI data itself, although the limit on the size of the UUI data (protocol discriminator plus 128 octets) will restrict this. 3.2. Impacts of the ISDN service on SIP operation The ISDN service has the following impacts that need to be understood within the SIP environment. Call transfer: ISDN call transfer cancels all User to User supplementary services. In the ISDN, if User to User data is required after call transfer, then UUS3 has to be renegotiated, which is not provided by this SIP extension. The impact of this restriction on the SIP environment is that UUI header fields cannot be exchanged in transactions clearing down the SIP dialog after call transfer has occurred. Conference: ISDN conferencing allows the user to still exchange User to User data after the conference is created. As far as UUS1 is concerned, it is not permitted. The ISDN three-party supplementary service is similar in many ways to conferencing, but is signalled using a different mechanism. This means that on Drage & Johnston Expires April 27, 2015 [Page 5] Internet-Draft ISDN Call Control UUI October 2014 clearing, the controller using UUS1 implicit does have the choice of sending data to either or both remote users. Because SIP conferencing cannot completely emulate the ISDN three-party supplementary service at the served user, UUS1 implicit is not possible. Diversion: When ISDN diversion occurs, any UUS1 User to User data is sent to the forwarded-to-user (assuming that the call meets requirements for providing the service - this is impacted by the explicit service only). If the type of diversion is such that the call is also delivered to the forwarding user, they will also receive any UUS1 User to User data. 4. Relation to SIP-T A method of transport of ISDN UUI is to use SIP-T [RFC3372] and transport the UUI information end-to-end, as part of an ISUP message or QSIG message) as a MIME body. If the SIP-T method of encapsulation of ISDN instead of interworking is used, this is a reasonable mechanism and does not require any extensions to existing SIP-T. However, if true ISDN interworking is being done, and therefore SIP-T would not otherwise be used, this approach is not reasonable, because then implementation of the many elements of ISUP syntax would be required to understand one element of data. Instead, the better approach is to interwork the ISDN UUI using the native SIP UUI transport mechanism, the User-to-User header field. The rest of this document describes this approach. 5. Transition away from ISDN This interworking usage of the SIP UUI mechanism will likely begin with one User Agent being an ISDN gateway while the other User Agent is a native SIP endpoint. As networks transition away from ISDN, it is possible that both User Agents could become native SIP endpoints. In this case, there is an opportunity to transition away from this ISDN usage to a more general usage of [I-D.ietf-cuss-sip-uui]. The SIP UUI mechanism provides a way to achieve this transition. As an endpoint moves from being an ISDN gateway to a native SIP endpoint, and a package for some form of enhanced UUI has been standardized, the endpoint can carry the UUI data both as ISDN and as some other package in parallel, and in the same messages or in different messages depending on the needs of the application. This will permit the other endpoint to use the UUI according to the ISDN package if it is an ISDN gateway or the enhanced package if it is a native SIP endpoint. Drage & Johnston Expires April 27, 2015 [Page 6] Internet-Draft ISDN Call Control UUI October 2014 6. ISDN Usage of the User-to-User Header Field This document defines the package for the ISDN interworking of UUI which is to interoperate with ISDN User to User Signaling (UUS), a supplementary service in which the user is able to send/receive a limited amount of information to/from another ISDN user over the signalling channel in association with a call to the other ISDN user. Two examples of ISDN UUI with redirection (transfer and diversion) are defined in [ANSII] and [ETSI]. One objective of the design of this package has been to keep the functionality at the interworking point as simple as possible. As a result there is also only one encoding value specified. Responsibility for respecting the limits has been transferred to the end UA. If an interworking point is reached, and the limitations of the ISDN (see section 3.1) are not met, then the UUI data will not be transferred, although the SIP request will otherwise be interworked, rather than have the interworking point attempt to resolve the non- compliance with the limitations of ISDN. The general principals of this package of the UUI mechanism are therefore as follows: That the sending application is expected to limit their sending requirements to the subset provided by the ISDN UUI service. That the SIP UA will not allow the reception of more that one User-to-User header field relating to the "isdn-uui" package in the same SIP request or response, and will only allow it in a request or response of the appropriate method (INVITE or BYE). What happens to User-to-User header fields relating to other packages is outside the scope of this document. That an interworking point trying to interwork UUI data that is too long will discard the UUI data, but proceed with the interworking. There is no notification of such discard back to the sending user. If the SIP user knows that it is interworking with the ISDN, then the UUI application at the SIP endpoint should limit its communication to 128 octet packets plus the protocol discriminator, in the knowledge that discard will occur if it does not. The UUI application at the SIP endpoint has complete control over what occurs. It should be noted that this was exactly the envisaged operation when early ISDN implementations that only supported 32 octets interworked with those supporting 128 octets. It also corresponds to the interworking with ISDNs that do not support the supplementary service at all, as discard will occur in these circumstances as well. Note that failure to include the Drage & Johnston Expires April 27, 2015 [Page 7] Internet-Draft ISDN Call Control UUI October 2014 User to User data into the ISDN SETUP message (when discard occurs) will result in the service being unavailable for the remainder of the call when UUS1 implicit operation is used. 7. UAC requirements The UAC MUST meet the requirements of [I-D.ietf-cuss-sip-uui] in addition to the requirements defined in this document. The UAC MUST only use this package of the UUI mechanism extension in association with the initial INVITE method and the BYE method relating to an INVITE dialog. Usage on transactions associated with any other type of dialog, or on methods not associated with a dialog is precluded. Usage on other methods within the INVITE dialog, and on re-INVITE transactions with the INVITE dialog, is also precluded. If the UAC wishes to use or permit the sending of UUI data at any point in the dialog, the UAC MUST include in the INVITE request for that dialog a User-to-User header field. The UAC SHOULD set the "purpose" header field parameter to "isdn-uui". Non-inclusion of the "purpose" header field parameter is permitted, but this is primarily to allow earlier implementations to support this package. This initial header field constitutes the implicit request to use the UUI service, and is therefore included even when there is no data except the protocol discriminator octet to send at that point in time. The UAC MUST NOT include the User-to-User header field with a "purpose" header field parameter set to "isdn-uui", or with no "purpose" header field parameter", in any message of an INVITE dialog if the original INVITE request did not include the User-to-User header field, either with a "purpose" header field parameter set to "isdn-uui", or with no "purpose" header field parameter included. When sending UUI for the ISDN package, if the "purpose" header field is included, the UAC MUST set the User-to-User "purpose" header field parameter to "isdn-uui". The UAC MUST NOT include more than one User-to-User header field for this package in any SIP request or response. When receiving UUI, when multiple User-to-User header fields are received in the same response with the "purpose" header field parameter to "isdn-uui", or with no "purpose" header field parameter, or with some combination of these, the UAC MUST discard all these header fields. There are no mechanisms for determining which was the intended UUI data so all are discarded. The application designer will need to take into account the ISDN Drage & Johnston Expires April 27, 2015 [Page 8] Internet-Draft ISDN Call Control UUI October 2014 service restrictions; failure to do so can result in information being discarded at any interworking point with the ISDN. This document makes no further normative requirements based on those constraints, because those constraints may vary from one ISDN to another. It is reasonable to expect that a limitation of 128 octets (plus a protocol discriminator) can be imposed by the ISDN, and therefore UUI data longer than this will never reach the destination if such interworking occurs. Note that the 128 octet limit (plus a protocol discriminator) applies before the encoding (or after the decoding) using the "hex" encoding. The "hex" encoding is defined in [I-D.ietf-cuss-sip-uui]. [I-D.ietf-cuss-sip-uui] defines a "uui" option tag for use with the UUI mechanism extension. Because for the ISDN UUI service, the service is UUS1 implicit, the inclusion of the "uui" option tag in a Supported header field conveys no additional information over and above the presence, in the INVITE request, of the User-to-User header field with the "purpose" header field parameter set to "isdn-uui". While there is no harm in including the "uui" option tag, and strictly it should be included if the extension is supported, it performs no function. The presence of the "uui" option tag in the Require header field of an INVITE request will cause the request to fail if it reaches a UAS or ISDN interworking gateway that does not support this extension; such a usage is not precluded although it does not form part of the package. 8. UAS requirements The UAS MUST meet the requirements of [I-D.ietf-cuss-sip-uui] in addition to the requirements defined in this document. The UAS MUST only use this package of the UUI mechanism extension in association with the initial INVITE method and the BYE method relating to an INVITE dialog. Usage on transactions associated with any other type of dialog, or on methods not associated with a dialog is precluded. Usage on other methods within the INVITE dialog, and on re-INVITE transactions with the INVITE dialog, is also precluded. The UAS MUST NOT include the User-to-User header field with a "purpose" header field parameter set to "isdn-uui", or with no "purpose" header field parameter", in any message of an INVITE dialog if the original INVITE request did not include the User-to-User header field, either with a "purpose" header field parameter set to "isdn-uui", or with no "purpose" header field parameter included. The UAS MAY include the User-to-User header field in responses to the initial INVITE request, or the BYE requests or responses for the Drage & Johnston Expires April 27, 2015 [Page 9] Internet-Draft ISDN Call Control UUI October 2014 dialog, only where the original INVITE request included a User-to- User header field with the "purpose" header field parameter set to "isdn-uui", or where no "purpose" header field parameter was included. When sending UUI for the ISDN package, the UAS SHOULD set the User-to-User "purpose" header field parameter to "isdn-uui". Non-inclusion of the "purpose" header field parameter is permitted, but this is primarily to allow earlier implementations to support this package. When sending UUI for the ISDN package, if the "purpose" header field is included, the UAS MUST set the User-to-User "purpose" header field parameter to "isdn-uui". The UAS MUST NOT include more than one User-to-User header field for this package in any SIP request or response. The "isdn-interwork" value for purpose parameter was used in Internet-Drafts that have led to the publication of the present document. Although these documents had no other status than "work in progress", this value is implemented by some vendors. While not defined by this document, implementations could find it useful for interoperability purposes to support parsing and interpreting "isdn- interwork" the same way as "isdn-uui" when receiving messages. Where the UAS is acting as a redirect server, the UAS MUST NOT include the User-to-User header field in the header URI parameter in a 3xx response to an incoming request. When receiving UUI, when a User-to-User header field is received in a request that is not from the originating user with the "purpose" header field parameter to "isdn-uui", or with no "purpose" header field parameter, the UAS MUST discard this header field. When receiving UUI, when multiple User-to-User header fields are received from the originating user in the same request with the "purpose" header field parameter to "isdn-uui", or with no "purpose" header field parameter, or with some combination of these, the UAS MUST discard all these header fields. There are no mechanisms for determining which was the intended UUI data so all are discarded. 9. UUI contents These requirements apply when the "purpose" header field parameter is set to "isdn-uui", or with no "purpose" header field parameter. Processing for User-to-User header fields sent or received with values other than this value are outside the scope of this document, and the appropriate package document for that value applies. Drage & Johnston Expires April 27, 2015 [Page 10] Internet-Draft ISDN Call Control UUI October 2014 The default and only content defined for this package is "isdn-uui". When sending UUI, the sending SIP entity MAY, but need not, include a "content" header field with a value set to "isdn-uui". A receiving SIP entity MUST ignore a received User-to-User header field if the "content" header field parameter is present and the value is some other value than "isdn-uui". The default and only encoding defined for this package is "hex". When sending UUI, the sending SIP entity MAY, but need not, include an "encoding" header field with a value set to "hex". A receiving SIP entity MUST ignore a received User-to-User header field if the "encoding" header field parameter is present and the value is some other value that "hex". When sending UUI, the sending application MUST include a protocol discriminator octet, conforming to table 4-26 of ITU-T Recommendation Q.931 [Q931] as the first octet of the UUI data. It is up to the receiving application what it does with this value. This document places no other normative requirement on the use of the protocol discriminator; it is required at interworking gateways to allow mapping into the appropriate fields in the ISDN protocols, but otherwise the usage is entirely up to the application, and outside the scope of this document. Valid values are identified and documented by ITU-T, and there is no IANA registry for these values. 10. Considerations for ISDN internetworking gateways ISDN interworking gateways MUST support the requirements defined for UAS and UAC operation. ISDN interworking gateways MUST support only the "isdn-uui" package on dialogs that are interworked. ISDN interworking gateways will take octet structured data from the ISDN side and encode it using the "hex" encoding scheme defined in [I-D.ietf-cuss-sip-uui] for inclusion as the uui-data in the User-to- User header field. In the reverse direction, it will take valid uui- data according to the "hex" encoding scheme, and decode it to octet structured data for sending to the ISDN side. When mapping data content from the ISDN to the SIP signalling, or from SIP signalling to the ISDN, the gateway needs to assume that all content is octet structured binary, irrespective of the value of the received protocol discriminator. There are no requirements in the ISDN to ensure that the content matches the value of the protocol discriminator, and it is for the application usage to sort out any discrepancy. The same applies to the ISDN protocol discrimination Drage & Johnston Expires April 27, 2015 [Page 11] Internet-Draft ISDN Call Control UUI October 2014 defined table 4-26 of ITU-T Recommendation Q.931 [Q931] as the first octet of the UUI data; the interworking gateway will not perform any additional checking of this value. [I-D.ietf-cuss-sip-uui] defines a "uui" option tag for use with the UUI mechanism extension. The option tag is not interworked at an ISDN interworking gateway. The ISDN interworking gateways MUST NOT take the omission of the "uui" option tag in a received INVITE request to indicate that interworking of a received header field is not to be performed. 11. Coding requirements This document defines "isdn-uui" as a new value of the User-to-User "purpose" header field parameter. The following ABNF adds to the production in [I-D.ietf-cuss-sip-uui]: pkg-param-value =/ "isdn-uui" This document defines "isdn-uui" as a new value of the User-to-User "content" header field parameter. A content value of "isdn-uui" indicates that the contents have a first octet that is a protocol discriminator (see table 4-26 of ITU-T Recommendation Q.931 [Q931]) followed by uui-data that can be subject to a length limitation (before encoding or after decoding) that is generally 128 octets. The following ABNF adds to the production in [I-D.ietf-cuss-sip-uui]. cont-param-value =/ "isdn-uui" 12. Media Feature Tag This document defines a new media feature tag "sip.uui-isdn". This feature tag indicates that this UUI package is supported by the sender, and its usage is entirely in accordance with RFC 3840 [RFC3840]. This document makes no additional provisions for the use of this feature tag. 13. IANA Considerations This document adds the following row to the "UUI packages" sub- registry of the SIP parameter registry: Value: isdn-uui Drage & Johnston Expires April 27, 2015 [Page 12] Internet-Draft ISDN Call Control UUI October 2014 Description: The associated application is being used with constraints suitable for interworking with the ISDN User to User service, and therefore can be interworked at ISDN gateways. Reference: RFCXXXX Contact: Keith Drage This document adds the following row to the "UUI content" subregistry of the SIP parameter registry: Value: isdn-uui Description: The associated contents conforms to the content associated with the ISDN User to User service. In the presence of the "purpose" header field parameter set to "isdn-uui" (or the absence of any "purpose" header field parameter) this is the default meaning and therefore need not be included in this case. Reference: RFCXXXX Contact: Keith Drage This document defines the following media feature tag which is added to the features.sip-tree of the Media Feature tags registry: Media feature-tag name: sip.uui-isdn ASN.1 Identifier: 1.3.6.1.8.4.x Summary of the media feature indicated by this tag: This media feature-tag when used in a Contact header field of a SIP request or a SIP response indicates that the entity sending the SIP message supports the UUI package "uui-isdn". Values appropriate for use with this feature-tag: none Examples of typical use: Indicating that a mobile phone supports SRVCC for calls in alerting phase. Related standards or documents: RFCXXXX Security Considerations: Security considerations for this media feature-tag are discussed in section 11.1 of [RFC3840] Editor's Note: [RFCXXXX] should be replaced with the designation of this document. Drage & Johnston Expires April 27, 2015 [Page 13] Internet-Draft ISDN Call Control UUI October 2014 14. Security Considerations This document contains no specific requirements in regard to security over and above those specified in [I-D.ietf-cuss-sip-uui]. However, since this capability is designed to interwork with the ISDN, the general security considerations of SIP to ISUP (ISDN User Part) interworking defined in [RFC3398] apply. Any SIP/PSTN gateway implementing the ISDN UUI service should not blindly trust ISUP from the PSTN. In general, the overlying use case will define the security measures required. The underlying User to User extension provides a number of tools that can meet certain security requirements. Information that might otherwise reveal private information about an individual, or where a level of authenticity needs to be guaranteed, may need a higher level of protection, and may indeed not be suitable for this package, particularly taking into account the statement in the following paragraph. As this capability is defined to interwork with the ISDN, if the ISDN forms part of the route, any usage needs to be aware that the security level of the ISDN service may be lower than the security of the SIP service. The ISDN security is itself not definable on an end-to-end basis, and exists on a hop-by-hop basis. This can be high in some places (e.g. it can require physical access to a secure building) and in other places it can be low (e.g. the point where an ISDN access enters a building). If this level of security is not sufficient, then either a different User to User package, or indeed, a different method of data transfer, needs to be selected by the application user. 15. Acknowledgments Joanne McMillen was a major contributor and co-author of earlier versions of this document. Thanks to Spencer Dawkins, Vijay Gurbani, Laura Liess, and Roland Jesske for their reviews of this document. The authors wish to thank Francois Audet, Denis Alexeitsev, Paul Kyzivat, Cullen Jennings, Mahalingam Mani and Celine Serrut-Valette for their comments. The death of Francois Audet occurred before this document was finalised, and the authors would like to identify the significant contribution of Francois to this and a number of important RFCs, and to express their condolences to his family. It was always a pleasure to work with Francois. Drage & Johnston Expires April 27, 2015 [Page 14] Internet-Draft ISDN Call Control UUI October 2014 16. References 16.1. 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. [RFC3372] Vemuri, A. and J. Peterson, "Session Initiation Protocol for Telephones (SIP-T): Context and Architectures", BCP 63, RFC 3372, September 2002. [RFC3840] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Indicating User Agent Capabilities in the Session Initiation Protocol (SIP)", RFC 3840, August 2004. [I-D.ietf-cuss-sip-uui] Johnston, A. and J. Rafferty, "A Mechanism for Transporting User to User Call Control Information in SIP", draft-ietf-cuss-sip-uui-17 (work in progress), June 2014. [Q931] "ITU-T Recommendation Q.931: Digital subscriber Signalling System No. 1 - Network layer; ISDN user-network interface layer 3 specification for basic call control", ITU-T http://www.itu.int/rec/T-REC-Q.931-199805-I/en. [RFC3398] Camarillo, G., Roach, A., Peterson, J., and L. Ong, "Integrated Services Digital Network (ISDN) User Part (ISUP) to Session Initiation Protocol (SIP) Mapping", RFC 3398, December 2002. 16.2. Informative References [Q957.1] "ITU-T Recommendation Q.957.1: Digital subscriber Signalling System No. 1 - Stage 3 description for supplementary services using DSS 1; Stage 3 description for additional information transfer supplementary services using DSS 1: User-to-User Signalling (UUS)", ITU-T http://www.itu.int/rec/T-REC-Q.957.1-199607-I. [Q763] "ITU-T Q.763 Signaling System No. 7 - ISDN user part formats and codes", ITU-T http://www.itu.int/rec/T-REC-Q.931-199805-I/en. Drage & Johnston Expires April 27, 2015 [Page 15] Internet-Draft ISDN Call Control UUI October 2014 [RFC6567] Johnston, A. and L. Liess, "Problem Statement and Requirements for Transporting User-to-User Call Control Information in SIP", RFC 6567, April 2012. [ANSII] "ANSI T1.643-1995, Telecommunications-Integrated Services Digital Network (ISDN)-Explicit Call Transfer Supplementary Service", ANSI T1.643-1995 . [ETSI] ""ETSI ETS 300 207-1 Ed.1 (1994), Integrated Services Digital Network (ISDN); Diversion supplementary services"", ETSI ETF 300 207-1 . Authors' Addresses Keith Drage (editor) Alcatel-Lucent Quadrant, Stonehill Green, Westlea Swindon UK Email: keith.drage@alcatel-lucent.com Alan Johnston Avaya St. Loius, MO US Email: alan.b.johnston@gmail.com Drage & Johnston Expires April 27, 2015 [Page 16]