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
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Abstract
This document proposes the use of the Reason header field in SIP
responses.
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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
---------------- ------------
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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
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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 |
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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 |----------------->|
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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
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