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Internet Engineering Task Force (IETF)                         R. JesskeRequest for Comments: 6432                                      L. LiessCategory: Standards Track                               Deutsche TelekomISSN: 2070-1721                                            November 2011Carrying Q.850 Codes in Reason Header Fieldsin SIP (Session Initiation Protocol) ResponsesAbstract   Although the use of the SIP (Session Initiation Protocol) Reason   header field in responses is considered in general inRFC 3326, its   use is not specified for any particular response code.  Nonetheless,   existing deployments have been using Reason header fields to carry   failure-related Q.850 cause codes in SIP responses to INVITE requests   that have been gatewayed to Public Switched Telephone Network (PSTN)   systems.  This document normatively describes the use of the Reason   header field in carrying Q.850 cause codes in SIP responses.Status of This Memo   This is an Internet Standards Track document.   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).  Further information on   Internet Standards is available inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6432.Copyright Notice   Copyright (c) 2011 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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.Jesske & Liess               Standards Track                    [Page 1]

RFC 6432                   Reason Header Field             November 2011   This document may contain material from IETF Documents or IETF   Contributions published or made publicly available before November   10, 2008.  The person(s) controlling the copyright in some of this   material may not have granted the IETF Trust the right to allow   modifications of such material outside the IETF Standards Process.   Without obtaining an adequate license from the person(s) controlling   the copyright in such materials, this document may not be modified   outside the IETF Standards Process, and derivative works of it may   not be created outside the IETF Standards Process, except to format   it for publication as an RFC or to translate it into languages other   than English.Table of Contents1. Overview ........................................................22. Terminology .....................................................23. Applicability ...................................................34. Security Considerations .........................................35. Acknowledgments .................................................36. Normative References ............................................31.  Overview   Although the use of the SIP (Session Initiation Protocol) Reason   header field in responses is considered in general inRFC 3326   [RFC3326], its use is not specified for any particular response code.   Nonetheless, existing deployments have been using Reason header   fields to carry failure-related Q.850 [Q.850] cause codes in SIP   responses to INVITE requests that have been gatewayed to PSTN   systems.  This document normatively describes the use of the Reason   header field in SIP responses to carry Q.850 [Q.850] cause codes.2.  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].   This document uses terms from [RFC3261].Jesske & Liess               Standards Track                    [Page 2]

RFC 6432                   Reason Header Field             November 20113.  Applicability   This document allows SIP responses to carry Reason header fields as   follows:      Any SIP Response message, with the exception of a 100 (Trying),      MAY contain a Reason header field with a Q.850 [Q.850] cause code.      The Reason header field is not needed in the 100 (Trying)      responses, since they are transmitted hop by hop, not end to end.      SIP responses with Reason header fields carrying values other than      Q.850 [Q.850] cause codes are outside of the scope of this      document.4.  Security Considerations   This specification allows the presence of the Reason header field   containing Q.850 [Q.850] cause codes in responses.  The presence of   the Reason header field in a response does not affect the treatment   of the response.  Nevertheless, there could be situations where a   wrong Q.850 [Q.850] cause code could, for example, cause an   announcement system to play the wrong information.  To avoid such   situations, it is RECOMMENDED that this header field be protected by   a suitable integrity mechanism.  The use of transport- or network-   layer hop-by-hop security mechanisms, such as Transport Layer   Security (TLS) or IPsec with appropriate cipher suites, can satisfy   this requirement.5.  Acknowledgments   Thanks to Gonzalo Camarillo and Mary Barnes for the detailed review   of this document.   Thanks to Paul Kyzivat, Mary Barnes, John Elwell, Keith Drage, and   Thomas Belling, who provided helpful comments, feedback, and   suggestions.6.  Normative References   [Q.850]    "Usage of cause and location in the Digital Subscriber              Signalling System No. 1 and the Signalling System No. 7              ISDN User Part", ITU Recommendation Q.850, May 1998.   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119, March 1997.Jesske & Liess               Standards Track                    [Page 3]

RFC 6432                   Reason Header Field             November 2011   [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.   [RFC3326]  Schulzrinne, H., Oran, D., and G. Camarillo, "The Reason              Header Field for the Session Initiation Protocol (SIP)",RFC 3326, December 2002.Authors' Addresses   Roland Jesske   Deutsche Telekom   Heinrich-Hertz-Strasse 3-7   Darmstadt  64307   Germany   Phone: +4961515812766   EMail: r.jesske@telekom.de   Laura Liess   Deutsche Telekom   Heinrich-Hertz-Strasse 3-7   Darmstadt  64307   Germany   Phone: +4961515812761   EMail: L.Liess@telekom.deJesske & Liess               Standards Track                    [Page 4]