Internet-Draft Reason Header in Responses February 2008 SIPPING Roland Jesske INTERNET-DRAFT Deutsche Telekom Intended Status: Informational Document: draft-jesske-sipping-etsi-ngn-reason-03.txt Expires: August 18, 2008 February 19, 2008 Use of the Reason header filed in Session Initiation Protocol (SIP) responses draft-jesske-sipping-etsi-ngn-reason-03.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of 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- Drafts. 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 http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on August 18, 2008. Copyright Notice Copyright (C) The IETF Trust (2008). Abstract This document proposes the use of the Reason header field in SIP responses. Jesske Expires - August 2008 [Page 1] Internet-Draft Reason Header in Responses February 2008 Table of Contents 1. Overview.......................................................2 2. Overall Applicability..........................................3 3. Terminology....................................................3 4. Procedures.....................................................3 4.1 Procedures at the UA.......................................4 4.2 Procedures at a SIP proxy..................................4 4.3 Procedures at an application server........................4 5. Procedures at an interworking point with ISUP..................4 6. Example........................................................4 7. Security Considerations........................................6 8. IANA Considerations............................................6 9. References.....................................................6 1. Overview The European Telecommunication Standards Institute (ETSI) is defining a Next Generation Network (NGN) where a substantial part of it is based on the IP Multimedia Subsystem (IMS) defined by the Third- Generation Partnership Project (3GPP). IMS is largely based on the Session Initiation Protocol [1]. ETSI has developed a number of requirements draft-jesske-sipping- tispan-requirements [5] to support the usage of SIP in Next Generation Networks that interoperate, at the service level, with the Public Switched Telephone Network (PSTN), the Integrated Services Digital Network (ISDN), the 3GPP IP Multimedia Subsystem (IMS), and SIP networks and terminals that implement the service logic. In order to provide full support in SIP of existing services, extensions to SIP are needed. This document proposes the use of the Reason header field in responses. This is needed for creating services that must be interoperable with the PSTN/ISDN network and the interoperability of traversing communications through SIP not using SIP-I. RFC3398 and other Interworking specifications like 3GPP TS 29.163 [11] are describing the mapping of ISUP Cause Values to SIP and vice versa. Looking on the specific mapping shows that information will be lost when the call traverses ISUP without using SIP-T. Example: RFC 3398 [10] describes the mapping of following ISUP Causes to 503 and 408 like follows. ISUP Cause value SIP response ---------------- ------------ Jesske Expires - August 2008 [Page 2] Internet-Draft Reason Header in Responses February 2008 34 no circuit available 503 Service unavailable 38 network out of order 503 Service unavailable 41 temporary failure 503 Service unavailable 42 switching equipment congestion 503 Service unavailable 47 resource unavailable 503 Service unavailable 58 bearer capability not presently 503 Service unavailable Available 88 incompatible destination 503 Service unavailable 18 no user responding 408 Request Timeout The mapping back is shown as follows: Response received Cause value in the REL ----------------- ---------------------- 503 Service unavailable 41 Temporary failure 408 Request timeout 102 Recovery on timer expiry The Example with 503 shows that a couple of different ISUP Cause values are interworked to only one SIP response. With 408 the meaning of the release cause is changed when interworked back to ISUP. Also Services built on Cause 18 (e.G. a 2nd call attempt on an other number, this service is like a sequential forking) will not work. 2. Overall Applicability The SIP procedures specified in this document are foreseen for networks providing simulation services and/or interworking to the PSTN/ISDN. The document is describing the use of the Reason header in SIP responses. These procedures are only valuable if the reason contained in the element "protocol" is "Q.850". A inclusion of a SIP reason (protocol="SIP") is not helpful due to the fact that the response already provides the SIP reason. The Release Causes are described within ETSI EN300 485 [5] 3. Terminology The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this document, are to be interpreted as described in RFC 2119 [RFC2119]. 4. Procedures Jesske Expires - August 2008 [Page 3] Internet-Draft Reason Header in Responses February 2008 For providing services and PSTN/ISDN interoperability it MUST be possible to include Reason header fields with Q.850 Cause values. 4.1 Procedures at the UA A UA that supports the Reason header field can process the Q.850 Cause Value and display it or an equivalent text. The inclusion of a Reason header field by UA is only for 2B2 UA interworking with the PSTN/ISDN or providing services foreseen. 4.2 Procedures at a SIP proxy SIP proxies that receive a response containing a Reason header field is forwarding the response without changing the reason. A SIP proxy receiving a request that includes a Reason header field can route the request to an application server for further analysis and base services on it. Based on network policy a Proxy can remove a Reason header field send from a UAC. 4.3P rocedures at an application server An application server that receives a SIP request that contains a response including a Reason header MAY analyze the SIP Reason and base further procedures on this analyses. For Example the application server could use the reason for sending a announcement towards the originating entity of the Session. As an example the Anonymous Communication Rejection (ACR) service defined by ETSI Telecommunications and Internet converged Services and Protocols for Advanced Networking (TISPAN) 5. Procedures at an interworking point with ISUP For interoperability reasons the Q.850 Cause Value of a Release shall be mapped to the Reason Header. 6. Example Figure 1 shows the example of SIP interworking with the PSTN/ISDN. Cause #87 is sent when the connecting user is not member of a Closed User Group. A Gateway Proxy AS | IAM | | | |------------------>| INVITE | | | |----------------->| INVITE | Jesske Expires - August 2008 [Page 4] Internet-Draft Reason Header in Responses February 2008 | | 100 Trying |----------------->| | |<-----------------| 100 Trying | | | |<-----------------| | ACK SDP held | | | |<------------------| | 603 Decline | | | 603 Decline | Reason Q850 #87 | | | Reason Q850 #87 | | | REL Cause #87 | |<-----------------| | |<-----------------| | |<----------------- | | | | | | | | | | | | | | | Figure 1: ISUP-SIP Call Figure 2 shows the example where the SIP network is used as transit between PSTN/ISDN networks. This avoids that the Mapping back to the Q.850 cause within ISUP change the meaning of the reason for release of the call. A Gateway Gateway B | IAM | | | |------------------>| INVITE | | | |----------------->| IAM | | | 100 Trying |----------------->| | |<-----------------| 100 Trying | | | 603 Decline | | | | Reason Q850 #87 | REL Cause #87 | | REL Cause #87 | |<-----------------| | |<-----------------| | |<----------------- | | | | | | | | | | | | | | | Figure 2: Transit case Figure 3 shows the example where the SIP network puts an announcement towards the UAB. The AS sends an announcement with a specific text back. After some Time the Response will be sent back to the UA A and closes all open transactions. With this possibility the SIP user can informed with more specific information than only the Response code. A AS Gateway B | INVITE | | | |------------------>| INVITE | | | |----------------->| IAM | | | 100 Trying |----------------->| Jesske Expires - August 2008 [Page 5] Internet-Draft Reason Header in Responses February 2008 | |<-----------------| | | | 503 Decline | | | | Reason Q850 #41 | REL Cause #41 | | | |<-----------------| | Announcement |<-----------------| | |< ================ | | | | | | | | 503 after Timeout| | | |<----------------- | | | Figure 3: Call Release within the PSTN with an announce played within the SIP network 7. Security Considerations The presence of the Reason header in a response does not affect the treatment of the response. Including such a header by an untrusted entity could adulterate the reactions of the originating entities. E.G. sending back a cause value "87" can cause an announcement within the PSTN/ISDN saying that the call was rejected due to the Closed User Group service. Therefore it is RECOMMENDED to include the Reason header information in Responses only by trusted entities as it is described within RFC3325 [7] 8. IANA Considerations This document describes the use of the Reason header field described within RFC 3326 [2]. No additional SIP elements are defined within this document. Therefore, this document does not provide any action to IANA. 9. References [1] 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. [2] H. Schulzrinne, D. Oran, G. Camarillo, "The Reason Header for the Session Initiation Protocol (SIP)", RFC 3326. [3] Jesske, R., Alexeitsev, D., Garcia-Martin, M. "Input Requirements for the Session Initiation Protocol (SIP) in support for the European Telecommunications Standards Institute (ETSI) Next Jesske Expires - August 2008 [Page 6]